1. Field of the Invention
The present invention relates to a detrack detecting system for detecting deviation o the beam spot from the track in an optical recording/reproducing apparatus in which recording and reproducing of data are performed by irradiating an optical beam onto an optical disc.
2. Description of the Prior Art
There has been put into practical use an optical disc in which a recording medium such that the reflectance changes in response to the light is provided on a recording surface of the optical disc, thereby enabling information to be recorded and reproduced.
In such a writable optical disc, for instance, as shown in FIG. 1, a spiral track is divided into n sectors S.sub.c1, S.sub.c2, S.sub.c3, . . . , S.sub.c4 and address areas AD.sub.1, AD.sub.2, AD.sub.3, . . . in which track address data is recorded are formed in the head portions of the sectors. A laser beam which was modulated by recording data is irradiated onto the data recording areas of the sectors S.sub.c1, S.sub.c2, S.sub.c3, . . . , so that the temperature of that portion rapidly rises and recording dots ar formed onto the disc surface. By irradiating a laser beam which is weaker than that upon recording, the recorded data can be read out without destroying the recording dots.
The laser beam which is irradiated onto the optical disc as mentioned above is controlled in a manner such that, in the recording and erasing modes, the laser power which is a few times as large as that in the reading mode is supplied. Therefore, when a strong vibration is applied to the optical recording/reproducing apparatus in the recording or erasing mode and the tracking servo is made inoperative, a serious accident happens such that the data which has already been written is destroyed or data is written to a wrong track on the optical disc.
Therefore, as shown in FIG. 2, there is considered a method whereby an amplitude of a tracking error signal which is obtained from the reflected light of the beam spot which was irradiated onto the optical disc is compared with reference levels Et.sub.1 and Et.sub.2 and when the tracking error signal exceeding the reference levels is output, it is regarded that the optical recording/reproducing apparatus is in the detracking state and when the detracking state is detected in the recording mode, the writing mode is cancelled at this time point and the erroneous recording or erroneous erasure of data is prevented.
However, in the detrack detecting system as mentioned above, since the level of tracking error signal changes according to the kind of optical disc or the reflectance of the optical disc, there is a problem such that even if the tracking error signal level is compared with the reference levels, it is difficult to detect an accurate detrack position.